Here's a link. Only the green balls have logos on them.

Ping pong balls give a little insulation and reduce evaporation. Lids with holes cut in them are good too but they often ensure that all of the evaporation goes through the hole which can damage your circulator over time. Plus ping pong balls conform to any size/shape container.

Ps - sometimes when people link to Amazon they make it known that they're not making money off of that link. I don't know how you make money from that but I assure you I am not.